Evolved high speed packet access (HSPA) systems are currently being developed within the framework of the Third Generation Partnership Project (3GPP) Release 7. One feature of the 3GPP Release 7 is the possibility for a user equipment (UE) to receive user and/or control data via a high-speed downlink shared channel (HS-DSCH) not only in a Cell_DCH state but also in Cell_FACH, URA_PCH, Cell_PCH states.
In the high speed downlink packet access (HSDPA), a UE receives a packet, (i.e., MAC-hs protocol data unit (PDU) or MAC-ehs PDU), from a Node-B implementing a hybrid automatic repeat request (HARQ) technique. In the Cell_DCH state, the UE sends a positive acknowledgment (ACK) or a negative acknowledgment (NACK) to the Node-B after each HARQ transmission to indicate whether the UE received the packet successfully or not.
Because of the delay required by the UE to decode and then transmit this feedback, the Node-B transmits (or re-transmits) different packets after sending the packet but before receiving the corresponding feedback for continuous transmission of packets. Since the number of transmissions required for successful decoding at the UE varies from packet to packet, there is a possibility that the HARQ entity in the UE does not deliver packets in the same order as their respective initial transmissions from the Node-B. To alleviate this issue, the medium access control (MAC) layer in the UE performs reordering prior to delivery of the received packets to higher layers. The reordering is based on a transmission sequence number (TSN) in the MAC-hs header.
While in the Cell_DCH state, a UE determines whether an HS-DSCH transmission from the Node-B is destined to the UE by masking the cyclic redundancy check (CRC) of the high-speed shared control channel (HS-SCCH) with its unique, (i.e., UE-specific), HS-DSCH radio network temporary identifier (H-RNTI). However, while in the Cell_PCH, URA_PCH, or Cell_FACH states, the UE does not necessarily have a UE-specific H-RNTI. For instance, upon cell reselection, the UE does not necessarily know its UE-specific H-RNTI to use in the target cell when the UE receives a cell update confirm message. To resolve this problem, the network may use a common H-RNTI that all WTRUs may decode and use in-band signaling to identify the UE. In addition, a common H-RNTI may be needed to allow the network to broadcast messages, (e.g., broadcast control channel (BCCH) messages), to all WTRUs camping on a given cell.
Some problems arise when attempting to implement the reordering functionality with UE(s) receiving data via the HS-DSCH utilizing a common H-RNTI. A first problem is that a UE potentially delays delivery of data to higher layers because the reordering function waits for the arrival of packets that are not even destined to the UE.
Another problem occurs when the UE performs MAC-hs or MAC-ehs reset, (e.g., upon cell resection). After performing cell reselection, a UE initializes some variables related to reordering, (e.g., next_expected_TSN and RcvWindow_UpperEdge), during the MAC-ehs reset. When utilizing a common H-RNTI, however, the target cell cannot re-initialize the TSN value without affecting all other WTRUs already utilizing this common H-RNTI in that target cell. Therefore, the UE joining the target cell cannot rely on the re-initialization of the TSN to perform subsequent reordering. As a result, undesirable effects may occur. For instance, if the sequence number (SN) of the first received packet after the MAC-ehs reset happens to be within the initial receive window and below the initial value of next_expected_TSN, this packet will be discarded.
Prior art could result in excessive delays upon performing the cell update procedure when utilizing the HS-DSCH in a Cell_FACH state. Node-B transmissions over the high-speed channel that are made without knowledge of the UE identity, (i.e., using a common H-RNTI), present a difficulty for the support of reordering. The Node-B cannot use UE-specific TSNs since the identity of the intended receiver is not known at the MAC-ehs entity of the UE. Thus, a UE starting to listen to such transmissions has no knowledge of the next TSN to expect for in-sequence delivery.